The invention relates to exhaust pipes for use in an exhaust system for combustion engines, for example of motor vehicles.
It is known that end pipes of exhaust systems for combustion engines for example of motor vehicles should be long in order to have a positive effect on sound attenuation with respect to low frequencies. Since space is limited in vehicles, pipes are bent. For example a pipe may be bent and a part of the pipe may be integrated inside a muffler while the remaining part of the pipe may be arranged outside of the muffler. It is also known that pipes with small diameters may improve the low frequent sound attenuation but may raise a flow velocity in the tube. However, bent pipes and fast flow velocities in pipes cause flow noise. Therefore, in prior art systems cross sections of a pipe in a bend are increased to reduce flow velocity. However, this has a negative effect on the low frequent sound attenuation. In other systems dividing plates are inserted into a bend such that the diameters of the divided pipes are reduced. However, these are more difficult to manufacture and the increased wetted surface and the introduction of additional edges reduce the potential pressure loss reduction and/or flow noise reduction. From further prior art it is known that there is a relationship between pressure loss in pipe bends and flow noise. By selecting an R/D ratio of at least 1.5 or larger for a pipe bend, pressure loss and flow noise is optimized. Therein, R is the bend radius and D is the diameter of the pipe. For example in DE 102009 000 090 a pipe bend with a constant bend radius R, with an oval cross section and a reduced diameter D in the bending plane has a R/D ratio of 3 or larger. The diameter of the pipe in the plane perpendicular to the bending plane may be slightly increased to partly compensate for the otherwise reduced cross section. However, to achieve large R/D ratios a small diameter of the pipe or a large bend radius is required. The former causes high flow noise and the latter does not allow to manufacture sharp bends and thus no compact arrangement of a bent pipe.
The exhaust pipe described in DE 10 2009 000 090 or other prior art still has drawbacks. Especially, prior art exhaust systems generally do not take into account differences in a flow entering a bend or leaving a bend. Therefore, there is still need for an improved exhaust pipe. Especially, there is need for such an exhaust pipe which provides an optimized bend structure with regard to pressure loss and flow noise.